Contractile Reserve: Are We Beginning to Understand It?

Echocardiography during preload stress for evaluation of right ventricular contractile reserve and exercise capacity in pulmonary hypertension

OBJECTIVES

Pulmonary hypertension (PH) is characterized by marked and sustained elevation of pulmonary vascular resistance and pulmonary artery pressure, and subsequent right-sided heart failure. Right ventricular (RV) function and exercise capacity have been recognized as important prognostic factors for PH. Our aim was to investigate RV contractile reserve and exercise capacity during a leg-positive pressure (LPP) maneuver.

METHODS

The study population comprised 43 PH patients and 17 normal controls. All patients underwent echocardiography at rest and during LPP stress. Exercise capacity was assessed by 6-minute walk distance for PH patients. RV relative wall thickness was calculated from dividing by RV free wall thickness by basal RV linear dimensions at end-diastole. RV function was calculated by averaging peak speckle-tracking longitudinal strain from the RV free wall. RV contractile reserve was assessed as the difference in RV free wall strain at rest and during LPP stress. Changes in left ventricular stroke volume (ΔSV) during LPP stress were also calculated.

RESULTS

ΔSV and RV contractile reserve of PH patients were significantly lower than of controls (3.6 ± 6.0 mL vs 8.5 ± 2.3 mL, and 8.2 ± 11.9% vs 14.5 ± 6.6%; both P < 0.01). RV contractile reserve of PH patients with ΔSV <3.3 mL was significantly lower than of PH patients with ΔSV >3.3 mL (3.9 ± 13.2% vs 12.3 ± 8.9%; P = 0.02). ΔSV had also significant correlation with 6-minute walk distance (r = 0.42, P = 0.006). Multivariate regression analysis showed that RV relative wall thickness was an independent determinant parameter of ΔSV during LPP stress for PH patients (β = 3.2, P = 0.003).

CONCLUSIONS

Preload stress echocardiography in response to LPP maneuver, a noninvasive and easy-to-use procedure for routine clinical use, proved to be useful for the assessment of RV contractile reserve and exercise capacity of PH patients.

Myocardial inotropic reserve: An old twist that constitutes a reliable index in the modern era of heart failure

Current national and international guidelines, including those of the European Society of Cardiology, recognize that the assessment of prognosis should be a part of the standard management for patients with chronic heart failure (CHF). However, these same guidelines recognize the inherent difficulty of this process. A variety of factors contribute to this difficulty, including the varying etiology, frequent co-morbidity and, perhaps most importantly, huge inter-individual variability in the disease progression and outcome. Although CHF is chronic, it is also a condition in which significant proportions of patients experience apparently 'sudden' death, which almost certainly contributes to our difficulty in assessing individual patient prognosis. A useful tool for the risk stratification of heart failure patients is dobutamine stress echocardiography (DSE), which determines the myocardial viability in ischemic cardiomyopathy and myocardial contractile reserve in idiopathic cardiomyopathy.

Aberrant sialylation causes dilated cardiomyopathy and stress-induced heart failure

Dilated cardiomyopathy (DCM), the third most common cause of heart failure, is often associated with arrhythmias and sudden cardiac death if not controlled. The majority of DCM is of unknown etiology. Protein sialylation is altered in human DCM, with responsible mechanisms not yet described. Here we sought to investigate the impact of clinically relevant changes in sialylation on cardiac function using a novel model for altered glycoprotein sialylation that leads to DCM and to chronic stress-induced heart failure (HF), deletion of the sialyltransferase, ST3Gal4. We previously reported that 12- to 20-week-old ST3Gal4 (-/-) mice showed aberrant cardiac voltage-gated ion channel sialylation and gating that contribute to a pro-arrhythmogenic phenotype. Here, echocardiography supported by histology revealed modest dilated and thinner-walled left ventricles without increased fibrosis in ST3Gal4 (-/-) mice starting at 1 year of age. Cardiac calcineurin expression in younger (16-20 weeks old) ST3Gal4 (-/-) hearts was significantly reduced compared to WT. Transverse aortic constriction (TAC) was used as a chronic stressor on the younger mice to determine whether the ability to compensate against a pathologic insult is compromised in the ST3Gal4 (-/-) heart, as suggested by previous reports describing the functional implications of reduced cardiac calcineurin levels. TAC'd ST3Gal4 (-/-) mice presented with significantly reduced systolic function and ventricular dilation that deteriorated into congestive HF within 6 weeks post-surgery, while constricted WT hearts remained well-adapted throughout (ejection fraction, ST3Gal4 (-/-) = 34 ± 5.2 %; WT = 53.8 ± 7.4 %; p < 0.05). Thus, a novel, sialo-dependent model for DCM/HF is described in which clinically relevant reduced sialylation results in increased arrhythmogenicity and reduced cardiac calcineurin levels that precede cardiomyopathy and TAC-induced HF, suggesting a causal link among aberrant sialylation, chronic arrhythmia, reduced calcineurin levels, DCM in the absence of a pathologic stimulus, and stress-induced HF.

Genetic Dissection of Cardiac Remodeling in an Isoproterenol-Induced Heart Failure Mouse Model

We aimed to understand the genetic control of cardiac remodeling using an isoproterenol-induced heart failure model in mice, which allowed control of confounding factors in an experimental setting. We characterized the changes in cardiac structure and function in response to chronic isoproterenol infusion using echocardiography in a panel of 104 inbred mouse strains. We showed that cardiac structure and function, whether under normal or stress conditions, has a strong genetic component, with heritability estimates of left ventricular mass between 61% and 81%. Association analyses of cardiac remodeling traits, corrected for population structure, body size and heart rate, revealed 17 genome-wide significant loci, including several loci containing previously implicated genes. Cardiac tissue gene expression profiling, expression quantitative trait loci, expression-phenotype correlation, and coding sequence variation analyses were performed to prioritize candidate genes and to generate hypotheses for downstream mechanistic studies. Using this approach, we have validated a novel gene, Myh14, as a negative regulator of ISO-induced left ventricular mass hypertrophy in an in vivo mouse model and demonstrated the up-regulation of immediate early gene Myc, fetal gene Nppb, and fibrosis gene Lgals3 in ISO-treated Myh14 deficient hearts compared to controls.

Heart failure is the most common cause of morbidity and mortality in the aging population. Previous large-scale human genome-wide association studies have yielded only a handful of genetic loci contributing to heart failure-related traits. Using a panel of diverse inbred mouse strains, treated with a β-adrenergic agonist isoproterenol to mimic the heart failure state, we sought to uncover the contribution of common genetic variation in heart failure. We found that heart failure has a strong genetic component. We successfully identified 17 genome-wide significant loci associated with indices of heart failure. We showed that genetic variation in a novel gene Myh14 affects heart failure by altering the mechanical responses of heart muscles to isoproterenol-induced stress. Follow-up studies of this gene and additional candidate genes and loci should reveal potential mechanisms by which genetic variations contribute to heart failure in the general human population. Such insights may lead to improved diagnosis and tailor treatment based on the genetic makeup of individuals in the population.

Dobutamine stress for evaluation of right ventricular reserve in pulmonary arterial hypertension

Right ventricular contractile response to pharmacological stress in pulmonary arterial hypertension (PAH) has not been characterised. We evaluated right ventricular contractile reserve in adults with PAH using dobutamine stress echocardiography.

16 PAH patients and 18 age-matched controls underwent low-dose dobutamine stress echocardiography. Contractile reserve was assessed by the change (Δ; peak stress minus rest value) in tricuspid annular plane systolic excursion (TAPSE) and tricuspid annular systolic velocity (S′). A subgroup of 13 PAH patients underwent treadmill cardiopulmonary exercise testing for peak oxygen uptake (V′O2peak).

At rest, TAPSE and S′ were reduced in the PAH group compared with controls (1.7±0.4 versus 2.4±0.2 cm and 9.7±2.6 versus 12.5±1.2 cm·s−1, respectively; p<0.05). Contractile reserve was markedly attenuated in PAH compared to controls (ΔTAPSE 0.1±0.2 versus 0.6±0.3 cm and ΔS′ 4.6±2.8 versus 11.2±3.6 cm·s−1; p<0.0001). In the sub-group of PAH patients with preserved right ventricular systolic function at rest, contractile reserve remained depressed compared to controls. V′O2peak was significantly correlated with ΔS′ (r=0.87, p=0.0003) and change in stroke volume (r=0.59, p=0.03).

Dobutamine stress can reveal sub-clinical reduction in right ventricular contractile reserve in patients with PAH. A correlation with exercise capacity suggests potential clinical value beyond resting measurements.

Age exacerbates chronic catecholamine-induced impairments in contractile reserve in the rat

Contractile reserve decreases with advancing age and chronic isoproterenol (ISO) administration is a well-characterized model of cardiac hypertrophy known to impair cardiovascular function. This study evaluated whether nonsenescent, mature adult rats are more susceptible to detrimental effects of chronic ISO administration than younger adult rats. Rats received daily injections of ISO (0.1 mg/kg sc) or vehicle for 3 wk. ISO induced a greater impairment in contractile reserve [maximum of left ventricular pressure development (Δ+dP/dt(max))] in mature adult ISO-treated (MA-ISO) than in young adult ISO-treated rats (YA-ISO) in response to infusions of mechanistically distinct inotropes (digoxin, milrinone; 20-200 μl·kg(-1)·min(-1)), while basal and agonist-induced changes in heart rate and systolic arterial pressure (SAP) were not different across groups. ISO decreased expression of the calcium handling protein, sarco(endo)plasmic reticulum Ca(2+)-ATPase-2a, in MA-ISO compared with YA, YA-ISO, and MA rats. Chronic ISO also induced greater increases in cardiac hypertrophy [left ventricular (LV) index: 33 ± 3 vs. 22 ± 5%] and caspase-3 activity (34 vs. 5%) in MA-ISO relative to YA-ISO rats. Moreover, β-myosin heavy chain (β-MHC) and atrial natriuretic factor (ANF) mRNA expression was significantly elevated in MA-ISO. These results demonstrate that adult rats develop greater impairments in systolic performance than younger rats when exposed to chronic catecholamine excess. Reduced contractile reserve may result from calcium dysregulation, increased caspase-3 activity, or increased β-MHC and ANF expression. Although several studies report age-related declines in systolic performance in older and senescent animals, the present study demonstrates that catecholamine excess induces reductions in systolic performance significantly earlier in life.